Yes, MPD would have direct access to the music files, and wouldn't need slow network access to the files.
Does the NAS have a (digital) sound output? Can it play audio over, say, HDMI? If not, then the Pi may be a better choice for actual playback, but MPD on the NAS could still do the database scans (https://www.musicpd.org/doc/html/user.html#satellite).